The present invention relates broadly to electromagnetic interference (EMI) shielding enclosures, such as cases, housings, or parts thereof such as covers, for mobile, i.e., cellular telephone handsets and other electronic devices, and particularly to a coating for such enclosures which is applied in a colored layer to an interior surface of an enclosure part formed of a clear or otherwise optically transparent or translucent plastic, such layer being electrically-conductive or otherwise providing EMI shielding and also being visually perceptible through the exterior surface of the part to provide a cosmetic enhancement to the enclosure or enclosure part.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is interposed between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. The housing may be formed of a metal such as steel, aluminum, or magnesium, or alternatively, of a plastic or other polymeric material which is provided with a conductive coating generally applied across the interior surfaces of the housing. The coating may be an electrically-conductive paint, a conductively-filled, molded elastomeric layer, a metal foil laminate or transfer, or a flame-sprayed or other deposited metal layer. A conductive gasket may be used to provide electrical continuity between the coating layers applied to the various mating housing parts.
An integrated EMI shielding solution for electronics enclosures, further described in commonly-assigned U.S. Pat. No. 5,566,055 and in DE 19728839, involves the over-molding of the housing or cover with an conductive elastomer. The elastomer is integrally molded in a relatively thin layer across the inside surface of the housing or cover, and in a relatively thicker layer along the interface locations thereof providing both a gasket-like response for environmentally sealing the cover to the housing and electrical continuity for the EMI shielding of the enclosure. The elastomer additionally may be molded onto interior partitions of the cover or housing, or itself molded to integrally-form such partitions, providing electromagnetically-isolated compartments between potentially interfering circuitry components. Covers of such type are marketed commercially under the name Cho-Shield(copyright) Cover by the Chomerics EMC Division of Parker-Hannifin Corporation (Woburn, Mass.).
Conventionally, most conductive coatings formulated for EMI shielding applications naturally exhibit a brown or gray appearance as a result of their being loaded with silver or particles or another metal filler. As many housings, and particularly those for mobile telephones, are formed of a black plastic, a black pigment or dye may be added to the coating to match the coating color to the substrate color. Coatings of such type are marketed by the Chomerics EMC Division of Parker Hannifin Corporation, Woburn, Mass., under the tradename Cho-Shield(trademark) 2052, and by Spraylat Corporation, Mount Vernon, N.Y. Should another color for the handset be desired, a nonconductive paint may be applied to the external surfaces of the housing. Alternatively, a colored faceplate may be used.
As electronic devices such as mobile phone handsets continue to proliferate, it is believed that additional EMI shielding alternatives and options for handset cases and other enclosures would be well-received by the electronics industry.
The present invention is directed to an electromagnetic interference (EMI) shielded enclosure, such as a case, housing, or a part thereof such as a cover, for mobile telephone handsets and other electronic devices. More particularly, the invention relates to an electrically-conductive coating or other shielding layer for such enclosures which is applied in an opaque or other colored layer to an interior surface of an enclosure part formed of a clear or otherwise optically transparent or translucent plastic, such layer providing EMI shielding and also being visually perceptible through the exterior surface of the part to provide the enclosure or enclosure part with a colorful or otherwise enhanced cosmetic appearance.
In an illustrative embodiment, the shielding layer is provided as an electrically-conductive, water-borne or solvent-based paint or other coating which may comprise a polyurethane, acrylic, or other resin binder and an electrically-conductive filler such as silver, silver-plated copper, or other metal particles or mixtures thereof. Although the coating may be left to exhibit a natural brown or silver color, a pigment, dye, or other colorant preferably is added to mask the natural color and to imbue another color such as black, white, yellow, green, red, blue, orange, purple, or, indeed, any color in the spectrum. Other or additional fillers or additives may be used to render the coating fluorescent, phosphorescent, or reflective. The coating may be sprayed, dip-coated, brushed, rolled, or otherwise applied to one or more of the interior surfaces of the enclosure or a part thereof such as a cover, and then dried or otherwise cured to form an adherent film or other layer thereon. Prior to cure, the layer may be embossed or otherwise textured to further enhance its light transmitting or reflecting priorities.
In further accordance with the precepts of the present invention, the enclosure or enclosure part to which the shielding layer is applied is formed of a clear, transparent, or translucent plastic which may be an acrylic, acrylonitrile-butadiene-styrene (ABS), polyvinyl chloride (PVC), polyphenylene ether (PPE), polystyrene (PS), polycarbonate (PC), polyamide, nylon, polyolefin, or a copolymer or blend thereof. As a result, the coating layer remains visually perceptible through the exterior surface of the enclosure part
Advantageously, the shielding layer so formed provides EMI shielding across the interior surface of the enclosure while, at the same time, eliminating the need for an external cosmetic coating. Moreover, by virtue of its application to the interior surface of the enclosure or enclosure part, the shielding layer, which may be formulated to exhibit a multitude of colors, is generally protected from the scratches or other damages to which an exterior paint is susceptible. Further, such shielding layer, in combining EMI shielding and cosmetic functions, is economical and simplifies manufacture in eliminating the necessity of having to provide a separate cosmetic paint layer. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.